The invention is based on an apparatus as defined hereinafter. In a known apparatus for generating pre-injections for internal combustion engines, in particular Diesel engines (German Offenlegungsschrift No. 25 09 068), a fuel injection valve, although it is intended for graduated injection, that is, without a marked time interval or an intervening valve closure between a pre-injection and a main injection, is associated with a pre-injection piston that is preceded coaxially by a main injection piston that mechanically rests directly on the pre-injection piston.
The high fuel pressure produced by an injection pump acts upon the main injection piston, which undergoes a displacement counter to a spring pressure that has a feedback effect on the pre-injection piston, and this displacement causes the pre-injection piston to emit a corresponding pre-injection quantity, and immediately after a predetermined pre-injection stroke is exceeded opens up a connection with the injection line. Although the main injection piston and the pre-injection piston in the known apparatus are embodied as stepped, so that intrinsically a pressure intensification could occur, it is expressly arranged that a fuel quantity having a feedback effect on the main injection piston and located in an outlet-side pressure chamber of the main injection piston is supplied via (throttled) transverse conduits to a pressure reservoir, so that while a pressure intensification is avoided, the injection pressure in the initial stage of injection (i.e., during pre-injection) is absolutely the same as during the main stage, with the single difference that in the initial stage, smaller quantities are injected. In this known fuel injection valve having graduated injection, there is no possibility for providing a pause in injection between the various stages or injection, or for shutting off the pre-injection completely; that is, a direct transition between the two stages takes place, without a pressure change.
It is generally known (German Offenlegungsschrift No. 1 576 478, German patent No. 1 284 687, or Austrian patent No. 289 469) for a fuel injection valve for pre- and main injection to be associated with a small spring-biased pre-injection piston, typically disposed parallel to the nozzle needle, and to subject the high-pressure side of the fuel injection valve to fuel in such a way that initially the pre-injection takes place by the movement of the small pre-injection piston and then the main injection is performed, optionally after an injection pause, once a predetermined state of equilibrium is attained between the faces acted upon by fuel and the spring forces acting upon various control elements or valves. In these known fuel injection valves, there is typically either no pressure intensification or only such a slight one in the vicinity of the pre-injection that by opening up the high-pressure-side fuel connection no seemingly backward-oriented pressure relief is brought about in any case in the pre-injection region by connection of the injection line with the high-pressure side; in the present invention, this is of functionally decisive significance. Since the spring influences and pressure equilibriums vary in their performance because of aging, adaptation to them causes inaccuracies in the desired piston capacity, this capacity being definitive for the injection pause between the pre-injection and the main injection. Finally, in the known fuel injection valves having pre-injection and main injection, the pre-injection cannot be selectively shut off from outside merely by the action of electrical control means.
It is also known (German Offenlegungsschrift No. 34 25 460), but without the possibility of a shutoff of the pre-injection, to embody a differential piston acted upon by the supply pressure of the high-pressure injection pump, which piston in its function as a storage piston, after the end of supply of the pre-injection, executes a distancing stroke that is injection-free and forms an injection pause.